MICROWAVE SYSTEM DESIGN

CONSIDERATIONS

Misunderstanding of complete system

System will surely fail

Without a solid understanding of complete communications system from the transmitter’s modulator input to the receiver’s modulator output, including everything in between, and how the selection of various components, circuits, and specifications can make or break

an entire system, any wireless design will surely fail .

PAIF Mixer

ModData in

LONoise

LNAMixerIF Amp IF Data out

LO

Dem

Block Diagram of Simple One Way Microwave Communication System

Baseband: Data, Voice, Video,….etc BW?

Modulation: Digital or Analog

Transmitter Components: IF Filters Mixer (up conversion)Filter PA RF FiltersAntenna

Link Calculation:Satellite Terrestrial Radar Wireless Mobile,

…..etc Receiver Components: Antenna RF Filters LNA Mixer (down conversion) IF Filters IFAmplifier

Demodulator

Other Components:Control System & Power Supply Monitoring: Test and measuring components & Measuring tools

Receiver Calculations

Noise

(So/No)min

ReceiverTo demodulator Signal + Noise

Internal Noise

Noise is added into an RF or IF passband and degrades system sensitivity

The receiving system does not register the difference between signal power and noise power. The external source, an antenna, will deliver both signal power and noise power to receiver. The system will add noise of its own to the input signal, then amplify the total package by the power gain

Noise behaves just like any other signal a system processes

Filters: will filter noise

Attenuators: will attenuate noise

ExternalInternal

ManmadeNatural

• Switching equipment

• Power generating equipment.

• Ignition Noise

• Interference

• Atmospheric

• Cosmic

• Galactic

• Thermal Noise

• Shot Noise

•Gen./Recomb.

•Flicker Noise

(Mod. Noise)

…………….

Noise Sources

Thermal NoiseThe most basic type of noise being caused by thermal vibration of bound charges. Also known as Johnson or Nyquist noise.

R

T Vn = 4KTBR

R

Available noise power Pn = KTB

Where, K = Boltzmann’s constant (1.3810-23J/oK)

T Absolute temperature in degrees Kelvin

B IF Band width in Hz

At room temperature 290o K:

For 1 Hz band width, Pn = -174 dBm

For 1 MHz Bandwidth Pn = -114 dBm

Shot Noise:

Source: random motion of charge carriers in electron tubes or solid state devices.

Noise in this case will be properly analyzed on based on noise figure or equivalent noise temperature

Generation-recombination noise:

Recombination noise is the random generation and recombination of holes and electrons inside the active devices due to thermal effects. When a hole and electron combine, they create a small current spike.

Antenna Noise

In a receiving system, antenna positioned to collect electromagnetic waves. Some of these waves will be the signals we are interested and some will be noise at the same frequency of the received signal. So filters could not be used to remove such noise.Antenna noise comes from the environment into which the antenna is looking. The noise power at the output of the antenna is equal to KTaB. Ta is the antenna temperature. The physical temperature of the antenna does not influence the value of Ta.

The noise temperature of the antenna can be reduced by repositioning it with respect to sources of external noise

Assumptions

■ Antenna has no earth-looking sidelobes or a backtobe (zero ground noise)

■ Antenna is lossless ■ h is antenna elevation angle (degrees)

■ Sun not considered ■ Cool. temperate-zone troposphere

Equivalent Noise Temperature and Noise Figure

F = (S/N)i/(S/N)o

Ni = Noise power from a matched load at To =290 K;

Ni = KTo B.

F is usually expressed in dB

F(dB)=10 log F.

Noise Figure (F)

Two-portNetworkSi + Ni So + No

Te = No/KB, B is generally the bandwidth of the component or system

Te = To( F – 1)

To is the actual temperature at the input port, usually 290 K

R No

white noise

source R R

Te No

Equivalent Noise Temperature (Te)

If an arbitrary noise source is white, so that its power spectral density is not a function of frequency, it can be modeled as equivalent thermal noise source and characterized by Te.

Examples:

(1) the noise power of a bipolar transistor at 3 GHz is 0.001 pW for a 1-MHz bandwidth. What is the noise temperature?

Solution WN = KTB, T = WN/KB = 72.5 K

F of the transistor is 0.97 dB

(2) the noise power of a mixer at 20 GHz is 0.01 pW for a I MHz bandwidth. what is the noise temperature ?

  Solution WN = KTB, T = WN/KB = 725K

F = 5.44 dB

Noise Figure of Cascaded Components

Te = To (F - 1)

Ts = Ta + Te Pn = KTsBG,

where, G is the overall gain of the system

F2 – 1

G1

F3 – 1

G1 G2

Fn – 1

G1 G2 ….. Gn-1

FT = F1 + + + …… +

F1

G1

F2

G2

FN-1

GN-1

FN

GN

Noise Figure of Passive and Active Circuits

Passive Components:

For Matching component:

F = L (L Insertion Loss)

Te = To (L-1)

F Increases if the component is mismatched.

Active Devices:

It is generally easier and more accurate to find the noise characteristics by direct measurement

Conversion Noise

Noise Free signal and Local Oscillator:

-10 dBm

-130 dBm

-17.5 dBm

-130 dBm

IL=7.5 dBF=7.5 dB

LO

RFIF

KTB = -130 dBm

17 dBm

-130 dBmNoise Figure = conversion loss

Noisy received signal:

17 dBm

-130 dBm

-17.5 dBm

-97.5 dBm

IL=7.5 dBF=7.5 dB

-10 dBm

-90 dBm

-130 dBmLO

RF IF

Noisy Local Oscillator:

-10 dBm

-130 dBm

IL=7.5 dBF=7.5 dB -17.5 dBm

-97.5 dBm

-130 dBm

17 dBm

-63 dBm

LO

RFIF

Noise Figure = 40 dB

Example: FT? Ts ? No ? Given IF bandwidth = 10 MH

NoiseLNAMixer

LO

BPF

G = 10 dB

Ta = 15 KF = 2 dBL = 1 dB

L = 3 dBF = 4 dB

So , No

Si , Ni

1) dB to numerical valuesLNA G = 10 dB (10) BPF: G = -1 dB (0.79) Mixer: G = -3 dB (0.5)

F = 2 dB (1.58) F = 1 dB (1.26) F = 4 dB (2.51)

2) FT = [ 1.58 + 0.26/10 + 1.51 /7.9] = 1.8 (2.55 dB)

3) Te = To(F-1) = 290 (1.8 – 1) = 232 K

4) Ts = Ta + Te = 247 K5) No = KTsBG,

G is the overall Gain = G1×G2×G3×….=10 × 0.79 × 0.5 = 3.95 (~6dB)No = -98.7 dBm

Dynamic Range, and 1-dB Compression Point

Input power

1 dB compression point

Output

power

1 dB

Dynamic rangeNoise floor

Minimum Detectable Signal (MDS)

MDS is dependent of the type of modulation used in receiving systems as well as the noise characteristics of the antenna and receiver. For a given system noise power, the MDS determines the minimum signal to noise ratio (SNR) at the demodulator of the receiver. The usable SNR depends on the application, with some typical values below

System SNR (dB)

Analog telephone 25-30

Analog television 45-55

AMPS cellular 18

QPSK (Pe = 10-5) 10

Ci/Ta Can be measured immediately following the receiver

Detector: Removes the signal from the carrier

S/N Can be determine

Noise

(Co/No)min

Receiver

TeFG

To demodulatorCi & Ta

Example: FM modulated signal 

SNR = C/No - 10 log B + 20 log (fu/fmax) + q w (dB) 

Where C/No = carrier to noise density (dBHz)B = channel bandwidth (Hz)

fu = test tone deviation at 0 dBm (Hz)

fmax = maximum frequency of baseband (Hz)

qw = combined psophometric and preemphasis factors (dB)  

Sensitivity: (MDS) 

Receiver voltage sensitivity, usually shortened to simply the receiver sensitivity.

 

Vimin = (2ZoSimin)0.5

Receiver Dynamic range: DRr = (maximum allowable signal power) / MDS

Defined by the third-order intercept point

Automatic Gain Control (AGC)

Why?DR(at the output of the receiver) < DR(at the input)Avoid receiver non-linearity

Receiver Gain: should be distributed throughout the RF, IF, & Baseband to avoid

non-linearity of the RF stage and take advantage of low cost IF amplifiers

G ~ 80-100 dB.

Input and Output Receiver Dynamic Range

Pr(dBm)

0

-20

-40

-60

-80

-100

-120

DRr

DRoutReceiver Gain

G

Low gain

High gainCi

Pb (V)

1

0.1

0.01

0.001

Pb

~ 80-100 dB

~ 60 dB

IF AGC circuit

IF Amp

Demodulator

LPF

Variable gain amp/attenuator

IF Input

DC AmpDC Ref

AGC detector

AGC Distributed Between RF and IF

IFdetector

IF amplifierFilter LNA MixerFilter

LPF

Comparator

Selection of IF frequency:

fIF = |fRF - fLO|

For lower side band selection

fLO = fRF + fIF

Frequency Conversion and Filtering

Large IF eases the cutoff requirements of the image filter

FIF > BRF/2 Image frequencies outside RF BW

IF < 100 MHz Low cost

fLO

fRF Image

IF IF

Transmitter

Radiate electromagnetic signal

Output:

Desired signal power

Harmonic

Spurious outputs

Wideband noise and phase noise,

Critical parameters:

Frequency and amplitude stability

Signal’s peak and average powers

Transmitted noise will raise the noise floor of the receiver

Link Budgets

Tx Rx

Baseband signal

Baseband output

R

PtPr

GtGr

Pt is the transmitted power

Gt is the transmit antenna gain

Gr is the receive antenna gain

Pr are the received power

The power density radiated by an isotropic antenna at a distance R is given by

  Savg = Pt/4R2 W/m2

The power density radiated by the given antenna is

 

Savg = Pt Gt /4R2 W/m2

 

The received power will be

Pr = Savg Ae Pt Gt Ae/4R2 W

  Ae = Gr2/4 m

 The received power can be expressed as

  Pr = Pt Gt Gr2 /(4R)2 W

Pr / Ni = (Pt Gt) [2 /(4R)2] Gr / KTAB

= (Pt Gt) [2 /(4R)2] (Gr /TA)/KB  

= EIRP Path loss Figure of merit / KB where,

EIRP is the equivalent isotropic radiated power 

TA is the antenna noise temperature 

G/T is a useful figure of merit for a receive antenna because it characterizes the total noise power delivered by the antenna to the input of a receiver.

The power density of the transmitted wave at the target location is Wt

Wt = Pt Gt(q,f)/4pRt2 W/m2

RCA Radar cross section area (echo area). It depends on the angle of incidence, on the angle of observation, on the shape of the scatterer, on the EM properties of the matter that it is built of, and on the wavelength.

Some Other Microwave Systems Baseband Microwave Radio

RF Multiplexing technique

Baseband Repeater

Types of Microwave Devices

Passive DevicesNo DC Power & No Electronic control

Active DevicesUses DC Power or No Electronic control

Duplexers Diplexers Filters Couplers Bridges Splitters Dividers Combiners Isolators Circulators Attenuators Cables Adapters Delay lines TL

Waveguides Resonators R, L, C’s

Dielectrics Antennas

Opens, shorts, loads

Switches Multiplexers Mixers

Samplers Multipliers Diodes

Transistors Oscillators Amplifiers RFICs MICs

MMICs Modulators VCOs

VTFs VCAtten’s VCAs

Tuners Converters Synthesizer